refactor: Enhance docs, code, add tests in HarshadNumber

[Hardvan]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.77%. Comparing base (219cc33) to head (39a6969).

Files with missing lines	Patch %	Lines
...in/java/com/thealgorithms/maths/HarshadNumber.java	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## master #6745 +/- ## ============================================ - Coverage 77.78% 77.77% -0.01%  - Complexity 6302 6304 +2  ============================================ Files 728 728 Lines 20986 20992 +6 Branches 4091 4092 +1 ============================================ + Hits 16323 16326 +3  - Misses 4021 4022 +1  - Partials 642 644 +2 

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